Variable gain amplifiers are used for controlling the amplitudes of transmission signals and received signals in communication devices such as mobile phones. Variable gain amplifiers are also used in phase shifters for controlling the phases of signals. For example, in a vector-sum phase shifter, a variable gain amplifier is used to control the amplitudes of an in-phase signal and a quadrature signal having phases shifted by 90 degrees from each other, and add the resulting two signals to generate an output signal. Control of the respective gains of the in-phase signal and the quadrature signal enables control of the phase of the output signal.
Such a variable gain amplifier is disclosed in Patent Literature 1 (Japanese Patent Application Publication No. 2007-259297), for example. FIG. 7 is a schematic configuration diagram of a variable gain amplifier 100 of related art disclosed in Patent Literature 1. The variable gain amplifier 100 has a digitally controlled current-steering configuration. As illustrated in FIG. 7, the variable gain amplifier 100 includes a bipolar transistor Q1 for performing voltage-to-current conversion on a signal input from a signal input terminal 101 via a capacitor C1, a base-current supplying circuit 121 for supplying base current to the bipolar transistor Q1, a signal transmission circuit 111 including a plurality of transistors P1 to PM, a signal short circuit 110 including a plurality of transistors A1 to AM, and a gate-voltage control circuit 122 for selectively turning on or off the transistors P1 to PM, and A1 to AM. The signal transmission circuit 111 is connected between a collector terminal of the bipolar transistor Q1 and the signal output terminal 102, and the signal short circuit 110 is connected between the collector terminal and a supply line of power supply voltage VDD. In addition, a load resistor 113 is connected between the signal transmission circuit 111 and the supply line of the power supply voltage VDD. Furthermore, a ground potential VSS is applied to an emitter of the bipolar transistor Q1.
A current signal output from the collector terminal of the bipolar transistor Q1 is input to the signal transmission circuit 111 and the signal short circuit 110. The gate-voltage control circuit 122 is capable of controlling the total number (total sum of ratios W/L of gate widths W to gate lengths L) of transistors to be in a turned-on states among transistors P1 to PM, and A1 to AM by controlling the gate voltage of each of the transistors P1 to PM, and A1 to AM. When the number of transistors in the turned-on states in the signal transmission circuit 111 is represented by np, and the number of transistors in the turned-on states in the signal short circuit 110 is represented by na, a current gain Di can be determined by the following expression:Di=np/(np+na)=1/(1+na/np).
The gate-voltage control circuit 122 makes an impedance constant as the signal transmission circuit 111 and the signal short circuit 110 are viewed from the collector terminal of the bipolar transistor Q1, by performing control to allow the value of the denominator np+na of the middle member of the expression to be constant.